The present invention is directed to starved electrolyte batteries and, in particular to starved electrolyte batteries wherein the electrode plates are at least partially encapsulated within resilient fibrous mat separators that extend beyond the peripheral edges of the electrode plates to both encapsulate the electrode plates and provide electrolyte reservoirs external of the electrode plate stack.
Rechargeable batteries, such as sealed, starved electrolyte, lead/acid batteries, are commonly used as power sources in vehicles, aircraft, emergency equipment and the like. These batteries, which typically range in size from xe2x80x9cDxe2x80x9d or xe2x80x9cbeer canxe2x80x9d sized batteries to larger sized batteries, are either single or multi-cell batteries. Currently, each cell of a single cell or multi-cell starved electrolyte, lead/acid battery is defined by a sealed compartment which houses a cell pack that includes at least one porous, positive electrode plate, at least one porous, negative electrode plate, and at least one porous, relatively fragile, microfiber glass mat separator between the electrode plates. A sulfuric acid electrolyte within each cell is absorbed by the porous, microfiber glass mat separator(s) and the porous electrode plates. Thus, the separators used in starved electrolyte, lead/acid batteries are intended to function as both: separators between the positive and negative electrode plates of the cells to maintain the spacings between the positive and negative electrode plates and prevent the formation of short circuits within the cells; and reservoirs for retaining electrolyte within the cells between the positive and negative electrode plates.
Short circuits within the cells of starved electrolyte batteries can occur due to direct contact between the positive and negative electrode plates when the spacing between the electrode plates is not maintained or due to the formation of dendrites or moss shaped particles of the electrode materials between the positive and negative electrode plates. Over the service life of such batteries, the electrode plates repeatedly expand and contract due to changes in active material morphology and density produced by the chemical reactions within the cells producing the electrical energy. Thus, to maintain the spacing between the positive and negative electrode plates over the service life of such a battery and to prolong the service life of such a battery, the electrolyte carrying separators should be resilient to maintain contact with the electrode plates and prevent short circuits through plate to plate contact. In addition, the separators should be free of openings, formed in the separators either during their manufacture or through the handling of the separators and assembly of the battery cells, to prevent or inhibit the formation of short circuiting active material growths, sheddings or dendrites between the electrode plates through such openings over the service life of the batteries.
Short circuits can also occur in starved electrolyte batteries due to the collection of electrode plate sheddings within a battery cell external of the electrode plate stack. To prevent short circuits between the electrode plates within a cell, caused by sheddings from the electrode plates that collect in the battery cells external of the electrode plate stacks and between the peripheral edges of the electrode plates, the electrode plates (including the edges of the electrode plates in whole or in part) should be encapsulated within the separators.
Since the separators in starved electrolyte batteries, such as starved electrolyte lead/acid batteries, also function as electrolyte reservoirs, the capacity of such batteries is a function of both the porosity and surface areas of the electrode plates and the porosity and surface areas of the separators in contact with the surfaces of the electrode plates. Thus, to maintain the electrolyte between the positive and negative electrode plates and to maintain the major surfaces of the separators in contact with the surfaces of the electrode plates, the separators of such batteries should be resilient so that the separators continue to recover in thickness after the repeated expansion and contraction of the electrode plates over the service life of such batteries. It would also be beneficial for the separators to provide for the storage of additional electrolyte, in reservoirs external of the plate stacks, that can be drawn into the portions of the separators intermediate the electrode plates to increase the capacity and/or the cycle service life of the battery cells.
Currently, thin, light weight mats or papers of glass fibers, polymeric fibers and/or other fibers (e.g. mats or papers ranging from about 100 to about 450 grams per square meter, such as glass microfiber separator mats for batteries) are made in various wet laid processes. In these wet laid paper making processes, the fibers are manufactured by various processes and collected in bulk. The glass and/or polymeric fibers are then hydropulped into very short fibers and introduced into and dispersed in a water slurry which is stirred to cause the fibers to become thoroughly and randomly mixed with each other. The fibers are then deposited from the water slurry onto a conventional paper making screen or wire as in a Fourdrinier machine or a Rotoformer machine to form a matted paper. When intended for use as a battery separator, the matted paper is then processed through an acid bath to bond the fibers of the matted paper together. After the matted paper is formed and processed through the acid bath, the matted paper is dried and wound up into a roll or otherwise collected in a conventional manner for further processing, such as being cut into selected sizes for use as a battery separator.
These processes for forming thin, light weight matted paper, result in matted paper separators which, at least in part due to the relatively short lengths of the hydropulped fibers in the mat, exhibit only limited recovery after compression and low integrity. These matted paper separators may also have openings through which active material growths or dendrites can form between the electrode plates and unless these separators are formed into pockets or similar encapsulating means, these separators do not prevent sheddings from collecting in the battery cells which could cause short circuits between the electrode plates. Thus, batteries utilizing these matted paper separators, with their limited recovery, limited integrity, and limited ability to prevent the formation of active material growths or dendrites and to prevent the collection of sheddings within a cell, may experience premature failure and there has been a need for batteries wherein the above problems are minimized or eliminated.
The present invention relates to starved electrolyte batteries, such as starved electrolyte lead/acid batteries, which incorporate resilient fibrous mat separators in the cells of the batteries: as separators between the electrode plates; as electrolyte reservoirs for maintaining electrolyte between the electrode plates; and as a means for encapsulating the electrode plates including the peripheral edges of the electrode plates in whole or in part. The resilient fibrous mat separators are compressed between the major surfaces of the electrode plates in a battery cell and extend beyond the perimeters of the electrode plates where they expand due to their resilience to encapsulate one or more of the peripheral edges of the electrode plates to prevent the shedding of active material from the electrode plates which can cause short circuits within the battery cell and to hold additional electrolyte which may increase the capacity and/or cycle service life of the battery cell.
Preferably, the resilient, fibrous mats used in the batteries of the present invention are made of glass, polymeric, cellulose, and/or other fibers which exhibit good integrity. While the resilient, fibrous mats used in the separators of the present invention can be made of larger diameter fibers and of thicknesses greater than those preferred for the present invention, the resilient, fibrous mat separators used in the batteries of the present invention are preferably formed from thin air laid, layered fibrous blankets or mats of randomly oriented, entangled microfibers which minimize the presence of undesirable openings in the layered fibrous mats through which dendrites can form.
In one type of the preferred separators used in the starved electrolyte batteries of the present invention, the resilient fibrous mat separators are made from resilient fibrous mats of microfibers which include one or two relatively high density, high tensile strength fibrous surface layer(s) and a relatively low density, more resilient fibrous layer integral with and, in one embodiment, intermediate the two surface layers. Preferably, the mat is binderless and the microfibers in the surface layer(s) of the mat are more entangled than the microfibers in the resilient layer to provide the mat with greater integrity.
The resilient, multilayered fibrous mats used to form the separators of these first and second embodiments of the present invention are preferably formed from resilient, air laid fibrous blankets by subjecting one or both surfaces of the air laid blankets to hydroentanglement (using water or an acid solution as the liquid) to increase the entanglement of the fibers at and adjacent the major surface(s) of the blankets relative to the entanglement of the fibers in resilient fibrous layers of the blankets. The further entanglement of the fibers at and adjacent the major surface(s) of the blankets increases the tensile strength of the blankets at their surface(s) while retaining the resilience of the resilient fibrous layers within the blankets so that the resilient, layered fibrous separators formed from the blankets have good integrity and retain their resilience after being subjected to repeated compression and expansion cycles in sealed starved electrolyte batteries. After the surface layer or layers are formed, the blanket is dried to form a resilient, multilayered fibrous mat. The resilient fibrous mat separators are formed from these multilayered mats by cutting the resilient fibrous mats into the desired dimensions and shapes for the separators.
In another type of the preferred separators used in the starved electrolyte batteries of the present invention, the resilient fibrous mat separators are made from resilient air laid felted fibrous mats of randomly oriented microfibers which preferably have substantially uniform densities throughout their thicknesses. These resilient fibrous mats are made by forming air laid blankets of randomly oriented microfibers; flooding the air laid fibrous blankets with a liquid such as water or an acid solution; drawing a vacuum through the fibrous blankets to remove liquid from the fibrous blankets and set their thicknesses; and drying the fibrous blankets to form the resilient fibrous mats. The resilient fibrous mat separators are then formed from these multilayered mats by cutting the resilient fibrous mats into the desired dimensions and shapes for the separators.
The thicknesses and the resilience of the preferred resilient fibrous mat separators used in the starved electrolyte batteries of the present invention, not only keep the electrode plates properly spaced, maintain electrolyte intermediate and in contact with the electrode plates, encapsulate the edges of the electrode plates and create electrolyte reservoirs external to the cell plate stacks through their expansion (the xe2x80x9cmushroom effectxe2x80x9d), the thicknesses and the resilience of the preferred battery separators used in the starved electrolyte batteries of the present invention apply a more uniform pressure to the major surfaces of the electrode plates to keep the active material of the electrode plates from separating from the grids of the electrode plates and may also improve the ability of the cells to withstand vibrational conditions without appreciable damage to the electrode plate stacks. In addition, the flexibility of the preferred battery separators used in the starved electrolyte batteries of the present invention enables these battery separators to be folded about or wrapped about the edges of the electrode plates of a plate stack without tearing, fracturing or otherwise failing.